Carbohydrate metabolism

Digestion & Absorption of carbohydrates
Prakash Pokhrel

Digestion is a process involving the hydrolysis of large
and complex organic molecules of foodstuffs into smaller
and preferably water-soluble molecules which can be
easily absorbed by the GIT for utilization by the organism
Digestion of macromolecules also promotes the
absorption of fat soluble vitamins and certain minerals

The principal dietary carbohydrates are polysaccharides
(starch, glycogen) disaccharides (lactose, sucrose) &
monosaccharides (glucose, fructose)
The digestion of carbohydrates occurs in the mouth &
intestine
The hydrolysis of glycosidic bonds is carried out by a group
of enzymes called glycosidases

DISACCHARIDES
• Sucrose (glucose+fructose)
• Lactose (glucose+galactose)
• Maltose (glucose+glucose)

Digestion in mouth:
Saliva contains carbohydrate splitting enzyme salivary
amylase (ptyalin)
Action of salivary amylase (ptyalin):
It is α – amylase, requires Cl- ions for activation & optimum
pH 6.7 (6.6 to 6.8)
Salivary amylase hydrolyses α 1-4 glycosidic bonds of
polysaccharides, producing smaller molecules maltose,
glucose & trisaccharide, maltotriose

Salivary amylase action stops in stomach when pH falls to 3.0
Digestion in stomach:
No carbohydrate splitting enzyme in gastric juice
Some dietary sucrose may be hydrolysed to equimolar
amounts of glucose & fructose by HCL
Digestion in duodenum:
Food bolus in duodenum mixes with pancreatic juice
Pancreatic juice contains pancreatic amylase, similar to
salivary amylase

Action of pancreatic amylase:
It is an α-amylase, optimum pH 7.1, requires Cl- ions
It specifically hydrolyzes α1-4 glycosidic bonds & not on
a1-6 bonds
It produces disaccharides (maltose, isomaltose) &
oligosaccharides
The final digestion of di- & oligosaccharides to
monosaccharides primarily occurs at the mucosal lining
of the upper jejunum

Carried out by oligosaccharidases (e.g. glucoamylase acting
on amylose) and disaccharidases (e.g. maltase, sucrase,
lactase)
Digestion in small intestine:
Action of intestinal juice:
Intestinal amylase: It hydrolyses terminal a 1-4-glycosidic
bonds in polysaccharides & oligosaccharides, liberating free
glucose
Lactase: It is β-galactosidase, its pH range 5.4 to 6.0
Lactose is hydrolysed to glucose & galactose

Absorption of carbohydrates
The principal monosaccharides produced by the digestion
of carbohydrates are glucose, fructose and galactose
Glucose accounts for 80% of the total monosaccharides
The absorption occurs mostly in the duodenum & upper
jejunum of small intestine
Only monosaccharides are absorbed by the intestine
Absorption rate is maximum for galactose; moderate for
glucose; and minimum for fructose

Absorption rates
Cori study:
He studies the rate of absorption of different sugars from
small intestine in rat
Glucose absorption as 100, comparative absorption of other
sugars as
Galactose=110, Glucose=100, Fructose=43, Mannoase=19,
Xylose=15 & Arabinose=9
Galactose is absorbed more rapidly than glucose
Pentoses are absorbed slowly

Mechanism of absorption
Different sugars possess different mechanisms for
their absorption
Glucose is transported into the intestinal mucosal cells
by a carrier mediated and energy requiring process

Monosaccharides, the end products of carbohydrate
digestion, enter the capillaries of the intestinal villi
In the liver,
galactose &
fructose are
converted to
glucose.
Small intestine
Monosaccharides travel to
the liver via the portal vein.

Active transport mechanism
Glucose and Na+ share the same transport system (symport)
referred to as sodium dependent glucose transporter
The concentration of Na+ is higher in the intestinal lumen
compared to mucosal cells
Na+ moves into the cells along its concentration gradient &
simultaneously glucose is transported into the intestinal cells
Mediated by the same carrier system

Na+ diffuses into the cell and it drags glucose along with it
The intestinal Na+ gradient is the immediate energy source
for glucose transport
This energy is indirectly supplied by ATP since the re-entry of
Na+ (against the concentration gradient) into the intestinal
lumen is an energy requiring active process
The enzyme Na+-K+ ATPase is involved in the transport of
Na+ in exchange of K+ against the concentration gradient

SGluT: Sodium and glucose co-transport system at
luminal side; sodium is then pumped out

Oral rehydration therapy (ORT):
ORT is common treatment of diarrhoea
Oral rehydration fluid contains glucose & sodium
Intestinal absorption of sodium is facilitated by the
presence of glucose
Mechanism of absorption of galactose is similar to that of
glucose
Phlorozin blocks the Na+ dependent transport of glucose &
galactose

Glucose transporters
Glucose transporters GluT-1 to 7 have been described in
various tissues
GluT-2 & GluT-4 are very important
GluT-2:
Operates in intestinal epithelial cells
It is a uniport system & not dependent on Na+ ions
Glucose is held on GluT-2, by weak hydrogen bonds
After fixing glucose, changes configuration & opens inner
side releasing glucose

GluT-4:
Operates in the muscle & adipose tissue
GluT-4 is under control of insulin
Insulin induces the intracellular GluT-4 molecules to move
to the cell membrane & increases the uptake
Other “GluT” molecules are not under control of insulin
GluT-1 is present in RBCs & brain
Also present in retina, colon, placenta
It helps in glucose uptake in most of these tissues which
is independent of insulin

Glucose transporters
Transporter Present in Properties
GluT1
RBC, brain, kidney, colon,
retina, placenta
Glucose uptake in most of cells
GluT2
Surface of intestinal cells, liver,
β-cells of pancreas
Low affinity; glucose uptake in liver;
glucose sensor in β-cells
GluT3
Neurons, brain High affinity; glucose into brain cells
GluT4
Skeletal, heart muscle,
adipose tissue
Insulin mediated glucose uptake
GluT5
Small intestine, testis,
sperms, kidney
Fructose transporter; poor ability to
transport glucose
GluT7 Liver endoplasmic reticulum Glucose from ER to cytoplasm
SGluT Intestine, kidney Cotransport; from lumen into cell

Absorption of fructose:
Fructose absorption is simple
Does not require energy and Na+ ions
Transported by facilitated diffusion mediated by a carrier
Inside the epithelial cell, most of the fructose is converted
to glucose
The latter then enters the circulation
Pentoses are absorbed by a process of simple diffusion

Factors influencing rate of absorption
Mucus membrane:
Mucus membrane is not healthy, absorption will decrease
Thyroid hormones:
Increases absorption of hexoses & act on intestinal mucosa
Adrenal cortex: Absorption decreases in adrenocortical
deficiency, mainly due to decreased concentration of sodium
Anterior pituitary: It affects mainly through thyroid hormones

Insulin:
It has no effect on absorption of glucose
Vitamins:
Absorption is decreased in B-complex vitamins
deficiency-thiamine, pyridoxine, pantothenic acid
Inherited deficiency of sucrase & lactase enzymes
interfere with corresponding disaccharide absorption

Abnormalities of carbohydrate digestions
Defect in disaccharidases results in the passage of undigested
disaccharides into the large intestine
The disaccharides draw water from the intestinal mucosa by
osmosis and cause osmotic diarrhoea
Bacterial action of these undigested carbohydrates leads to
flatulence
Flatulence is characterized by increased intestinal motility,
cramps and irritation

The carbohydrates (di, oligo and polysaccharides) not
hydrolysed by α-amylase
The di & oligosaccharides can be degraded by the bacteria
present in ileum to liberate monosaccharides
During the course of utilization of monosaccharides by the
intestinal bacteria, the gases such as hydrogen, methane &
carbon dioxide-besides lactate and short chain fatty acids
are released & causes flatulence

Lactose intolerance
lactase (β-galactosidase) deficiency is the most common
disaccharidase deficiency in humans
lt is estimated that more than half of the world's adult
population is affected by lactose intolerance
Some infants may have deficiency of lactase & they show
intolerance to lactose, the milk sugar
Symptoms:
Diarrhoea, flatulence, abdominal cramps

Discussion:
Lactose of milk cannot be hydrolysed due to deficiency of
lactase
Accumulation of lactose in intestinal tract, which is
“osmotically active” & holds water, producing diarrhoea.
Accumulated lactose is also fermented by intestinal
bacteria which produce gas & other products, producing
flatulence & abdominal pain

Carbohydrate metabolism

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Carbohydrate metabolism